+1 at what Phil said.
I've done a lot of cave shooting in my younger years.
Back then in the caves I've been there was zero light.
Nowadays with light cleverly placed inside, the only problem is to have your camera steady for long exposure. There are some places where flash isn't permitted anyway. What Phil said is very pertinent, the harsh shadows would ruin your shot. We used back then dosens of magnesium bulbs carefuly placed and wired to a single switch and 7- 8 cameras on tripods with shutters opened on T. Setting up one shingle shot could take a day. Now you have nice lighting, you just need a wide angle lens and a tripod. Good luck.

I took some shots in the caves in Mallorca. As well as the advice already given be prepared to keep wiping the lens as the temperature changes and moisture can produce a lot of condensation. You may have to spend a few minutes in each location to allow the camera temperature to change to the ambient temperature.

Well, it was an interesting experience. The weather was average and being a weekend, many people had the same idea.
The tour was busy and a bit rushed, there was little opportunity to take photos from the preferred angles, it was also fairly cramped for space. Tripods were not allowed, there were too many people, no room, no time.
I used the 50 1.8 mostly and I am still continuing my love hate relationship with it, actually there is little love. . It front and back focuses depending on distance. My 4 year old helped out with the head torch when focus became difficult.
For this cave the 50 wasn`t wide enough and the shallow dof was annoying and wide open its not sharp at all.

Something like the 17-55 IS USM would possibly have been ideal or the Sigma 30 1.4.
Overall I am disappoint in the results. Judging on how easy it and consistent it is to get photos with the S95 and flash, I may well have been better served taking the 15-85 and the 430EX II.

I think a big part of the problem, besides lack of experience, is the existing cave lighting, there is a huge variation in lighting. Some areas are blown out and others too dark, then there is motion blur and out of focus pics and that doesn`t leave too many reasonable shots.
These are either straight jpg`s or simply processed with Canon`s DPP, like I said at the start interesting experience

you are right there, I did use the 15-85 at the start, and the IS was definitely useful. Just at the longer zoom the light drop off was too much and the cave got darker. Once I changed to the 50, I didn`t want to change lenses again while in the cave and really needed the extra light coming in.

I can see the benefit of a constant aperture zoom with is. 17-55 IS USM coming up.